The present invention relates generally to a scan reading method of detecting an edge or both edges of an original and performing a density auto-measurement. More particularly, to the scan reading method of automatically detecting the edge or edges of the original mounted on a platen of an image reader using a scanner, i.e., the top end and terminal of the original or both ends thereof in a scan direction of the scanner, and of automatically measuring a density of the original, especially densities of highlight and shadow portions of the original.
The image reader for scan-reading an image of the original mounted on the platen (the original mount) has been employed for a variety of image forming apparatuses such as a printing plate making apparatus, a copying apparatus, etc.
In such an image reader, the original on the platen is irradiated with the light of a light source extending in a one-dimensional direction, e.g., a fluorescent lamp and an LED array light source, or the light emitted from a light source and deflected in the one-dimensional direction, e.g., the laser beam emitted from a laser light source deflected by an light deflector. The reflected light therefrom is further reflected by a plurality of mirrors in predetermined directions and penetrates a projection lens. An image is formed on a solid-state imaging element such as a CCD or the like disposed in a predetermined position. This solid-state imaging element effects a photoelectric conversion into electric signals, and image data of the original are obtained in the form of, e.g., density signals.
The thus obtained original image data are subjected to a variety of processes in an image processing device of the above-described image forming apparatus such as the printing plate making apparatus. The original image data are thereafter transmitted to an image recording device, wherein light beams emitted from the laser or the like are modulated. The light beams modulated corresponding to the image data are reflectively deflected in a main scan direction by a light deflector such as a resonant scanner, a galvanometer mirror, etc. A recording material transported at a constant velocity in a subscan direction substantially orthogonal to the main scan direction is two-dimensionally scan exposed to the light beams, thus performing image recording. A reproductive image is thereby obtained.
In the image forming apparatus such as the copying apparatus and the printing plate making apparatus, a photosensitive material or a paper having a desired or predetermined cut size must be selected for obtaining the reproductive image having a desired or predetermined image forming size in accordance with scale factors (of equalization, enlargement and contraction) and a size of the original. Further, there arises a necessity for cutting the paper and the photosensitive material in the above mentioned cut size. For this reason, the user has hitherto manually inputted the size of the original or manually inputted the edges of the original by use of a digitizer when the size is not fixed. This is troublesome. For instance, the manual input is extremely troublesome, especially when the original is not fixed in terms of size. This causes the operability of the apparatus to be deteriorated.
To improve this situation, in the image forming apparatus which often uses the fixed-size original and paper as in a black and white copying apparatus, for instance, a multiplicity of sensors are attached to the scanner along the periphery of the platen or under the platen. Detected thereby is a size of the original to be placed on the platen which is carried onto the platen directly or through an automatic document feeder (ADF) installed on the platen. In this system, however, with an intention of increasing an accuracy of a detection result, a tremendously large number of sensors are required. This also presents a problem where it is difficult to accurately detect the edges of the unfixed-size original.
On the other hand, for the purpose of corresponding to the unfixed sizes, in some of the printing plate making appazatuses, the user inputs the edge of the original to the image reading device by depressing a slide button after sliding the slide button up to the edge of the original on the platen. Nevertheless, this operation still requires manual input by the user. This results in the problems where the operation is troublesome, and the operability is bad.
Further, in the image forming apparatus such as the copying apparatus and the printing plate making apparatus, a continuous tone (gradation) image of the original is, after being converted into electric signals by use of a solid-state imaging element such as a CCD or the like, reproduced on a photosensitive material such as a film, etc. In this case, noises probably enter the reproductive image depending on characteristics of the foregoing solid-state imaging element. If noises enter the reproductive image, the harshness and unevenness are produced in the reproductive image. The reproductive image may not fit for viewing. In particular, a human visual perception exhibits a property approximate to the logarithm with respect to the brightness. Hence, even when the same amount of noises enter a shadow portion with a high density and a highlight portion with a low density of the continuous gradation image of the original, a large noise level thereof is sensed in the shadow portion with the high density. This is a factor which adds to the deterioration of image quantity. Known for reducing such visible noises is an image processing method of averaging the image signals as electric signals.
In the case of inputting by averaging the whole density levels of the continuous gradation image of the original, the reduction in noises is attained. However, an inconvenience is caused, wherein a resolution in the highlight portion deteriorates. Namely, the human visual perception exhibits such a characteristic that a spatial resolving power is low in the shadow portion of the continuous gradation image of the original, whereas the resolving power is high in the highlight portion. Therefore, it follows that the visible noises are preferably reduced in the shadow portion, but the resolving power deteriorates in the highlight portion.
To cope with this, in the highlight and shadow portions of the image of the original, the noises are reduced in a broad range extending from the shadow portion with the high density to the highlight portion with the low density in the reproductive image by changing the method of image processing such as an averaging process of the image data obtained, a smoothing process (unsharpness process) and a sharpening process (sharpness process). The unevenness and harshness are thereby eliminated. Specifically, this intends to prevent both a deterioration of the sharpness of the edge on the high density side and unnaturalness to the image with a gradual variation in density. Additionally, for controlling a pictorial gradation of a finished copy, the user inputs the densities of the highlight and shadow portions or effects sampling of the image density by scanning the original. A density histogram is thereafter prepared, and the highlight and shadow densities are automatically detected.
In the conventional density auto-measurement, when preparing the histogram, the image data for a sheet of read original (and after "lines") or at least several lines, are at once taken into a buffer and a frame memory. The histogram is then prepared. For this reason, a tremendous memory is needed as a buffer for the image data. This brings about an increment in costs of the image processor and in turn of the image reader and the printing plate making apparatus.